The invention relates to an apparatus for reducing the friction between two bodies and, in particular, for reducing the friction between the sensor and the object to be measured of a thickness measuring apparatus.
In previous designs for reducing the friction, thickness sensors for plastic foils, for example, are coated with materials which glide well on these foils (e.g. PTFE, i.e., polytetrafluorethylene, and/or Teflon™), are executed as rolling thickness sensors (EP 01182423) or an air pocket is built up between the sensor and object to be measured (UK 1 436 698).
For measuring the thickness of foils on which bodies such as sensors glide very poorly, so-called matte foils or foils which are even sticky, for example, with EVA (ethylene vinyl acetate) in the outer layer contacting the sensor, a surface coating with almost ideal gliding material (e.g. PTFE) is also not sufficient. Rolling thickness sensors and extreme contact-sensitive thickness sensors with air cushions are complicated in construction and are distance-sensitive.
The thickness measurement of plastic foils for blown foil lines is carried out for procedural reasons, for flat foil lines for cost reasons ideally with a reflective measuring operation. Known measuring methods are those which function, for example, capacitively, optically or with ionizing radiation.
In all of these measuring methods the distance to the object to be measured has a certain influence on the measuring precision. The contacting measurement operation (distance 0) would be metrologically optimal. In certain limits the distance from sensor to foil can be measured and the measurement signals can be arithmetically compensated by computation (mathematically) (EP0801290, EP1681531).